Commonly known or commercially employed fluoropolymers include polytetrafluoroethylene (PTFE), copolymers of tetrafluoroethylene (TFE) and hexafluoropropylene (HFP) (FEP polymers), copolymers of TFE and perfluoroalkoxy copolymers (PFA polymers), copolymers of TFE and ethylene (ETFE polymers), terpolymers of TFE, HFP, and vinylidene fluoride (VDF) (THV polymers) and polymers of VDF (PVDF polymers). Commercially employed fluoropolymers also include fluoroelastomers and thermoplastic fluoropolymers. Fluoropolymers and their applications are further described, for instance, in “Modern Fluoropolymers”, edited by John Scheirs, John Wiley & Sons Ltd., 1997. The making of such fluoropolymers generally involves the polymerization of gaseous monomers, i.e. monomers that under ambient conditions of temperature and pressure exist as a gas. Several polymerization methods are known to produce fluoropolymers. Such methods include suspension polymerization as described in e.g. U.S. Pat. Nos. 3,855,191, 4,439,385 and EP 649,863; aqueous emulsion polymerization as described in e.g. U.S. Pat. Nos. 3,635,926 and 4,262,101; solution polymerization as described in U.S. Pat. Nos. 3,642,742, 4,588,796 and 5,663,255; polymerization using supercritical CO2 as described in JP 46011031 and EP 964,009 and polymerization in the gas phase as disclosed in U.S. Pat. No. 4,861,845.
Currently, the most commonly employed polymerization methods include suspension polymerization and aqueous emulsion polymerization. Aqueous emulsion polymerization often involves polymerization in the presence of a fluorinated surfactant, which is generally used to ensure the stabilization of the polymer particles formed. Accordingly, the emulsion polymerization process differs from suspension polymerization in which generally no surfactant is used and which generally results in substantially larger polymer particles than are formed in aqueous emulsion polymerization.
In aqueous emulsion polymerization, the polymerization speed, yield of polymer, comonomer incorporation, particle size and particle number are dependent on each other as well as on the level of emulsifier content. In order to optimize polymer yield, polymerization time and particle size, one can modify the pressure of gaseous monomer(s), the temperature, and the type and amount of fluorinated surfactant. For example, EP 612,770 describes the use of high amounts of fluorinated surfactant to increase the polymerization speed. Increasing the amount of fluorinated surfactant, however, has the disadvantage of increasing the cost of polymerization and may also be environmentally undesirable.
U.S. Pat. No. 4,864,006 describes an aqueous emulsion polymerization process wherein an aqueous micro-emulsion of a perfluoropolyether is added to a polymerization reaction. The micro-emulsion is characterized as a mono-phase solution that is stable in the long run without providing “dispersion energy”. It thus seems that the micro-emulsion discussed forms upon simple mixing of the components without the need to apply “dispersion energy” through the use of special emulsifying equipment. Accordingly, the type of surfactants that can be used are limited, as not all surfactants are able to form such a micro-emulsion. The limitations on the useful surfactants may further complicate the polymerization process. For example, as shown in the examples of U.S. Pat. No. 4,864,006, the micro-emulsion of the perfluoropolyether is formed using a perfluoropolyether that has a carboxylic acid group as a surfactant. Such surfactants may be expensive. Further, as shown in the examples of U.S. Pat. No. 4,864,006, the micro-emulsion forms at elevated temperatures.
EP 969,027 describes a process for making PTFE dispersions. In this patent application, a micro-emulsion of a perfluoropolyether obtained through emulsification with a perfluoropolyether surfactant is used to prepare the PTFE dispersion. Although small polymer particles are achieved in this process, it appears that large amounts of surfactant are used. The use of large amounts of surfactants is disadvantageous in that it makes the process more costly and may put environmental burdens on the process.
U.S. Pat. No. 5,895,799 describes a micro-emulsion polymerization of TFE to produce very small particle dispersions of PTFE in water. The polymerization of TFE itself proceeds in a micro-emulsion of liquid perfluorinated hydrocarbons. This micro-emulsion is formed by adding the perfluorinated hydrocarbon liquid and a fluorinated surfactant to water at a temperature and in proportions that result in a micro-emulsion. As is well known, micro-emulsions only form when the surfactant is used above its critical micelle concentration (CMC). See P. Lovell, “Emulsion Polymerization and Emulsion Polymers”, J. Wiley, 1997, pages 700-739. As a result, the process taught in U.S. Pat. No. 5,895,799 will generally require large amounts of surfactant, typically much higher than the amounts employed in conventional aqueous emulsion polymerization processes that use surfactants in amounts below the CMC.